The present invention is directed to analog-to-digital (ADC) converters and, more particularly, a very high-speed ADC is described for the level detection of, or equivalently, the data extraction from high-speed multilevel (ML) waveforms.
Complex modulation schemes are desirable to obtain increased spectral efficiency and, therefore, increased data throughput for high-speed communications systems. The extraction of data from the multi-Giga symbol per second (Gsym/s) ML waveforms can be difficult, but is critical for effective implementation of a high-speed communications system.
Conventional flash ADC""s are common in high-speed systems; however, these ADC""s suffer three shortcomings. First, they are based on conventional Gray codes that result in asymmetric decoder circuitry in the sense that the circuitry for decoding the simplest bit channel can be significantly more complex than the circuitry for the most complex bit channel.
Conventional Gray codes are described in U.S. Pat. No. 2,632,058, entitled xe2x80x9cPulse Code Communicationxe2x80x9d, which issued to Gray on Mar. 17, 1953. This reflected-binary code (now generally called a Gray code) described in the ""058 patent provides for improved performance of pulse coded communications based on encoding data in time vs. voltage.
With conventional Gray code analog to digital decoders, buffers are usually used to delay the result from the simplest bit channel to match the output delay of the most complex channel. The overall result is an ADC that has high complexity which translates into high power consumption and lower achievable speed.
The second shortcoming of flash ADC""s based on conventional Gray codes is that the error rate on each of the bit channels is skewed. This skew can reduce the performance of error correction mechanisms where it is assumed each bit channel has the same error probability.
The third shortcoming of conventional flash ADC""s is that they have uniformly spaced decoding thresholds. In many communication contexts, the received signal is distorted by signal dependent noise and the use of uniformly spaced thresholds is suboptimal.
In view of the foregoing, there is a need in the art for an ADC where the thresholds are independently and externally adjustable to provide a means for maintaining optimal decoding in the presence of (possibly time-varying) signal dependent distortions. There is a further need in the art for an ADC that can support the adjustable quantization of ML signals within the receiver of a high-speed telecommunication system. Another need exists in the art for an evenly distributed bit error rate across each of the bit channels in a multilevel signal. And further, a need exists in the art for a simpler ADC design for high speed communications.
A design for an analog-to-digital converter that can decode a unique multilevel Gray code can be made more simple than conventional analog to digital converters. More specifically, because of certain properties of the inventive and exemplary Q-Gray codes of the present invention, the analog-to-digital converter can comprise a plurality of comparators for receiving the multilevel signal and a plurality of decoder blocks coupled to comparators for decoding the multilevel signal. Each decoder block can comprise an equal number of inputs. Specifically, each decoder block can also comprise a parity detector with an equal number of inputs. Each decoder block can also comprise a bank of identical parity detectors relative to another decoder block.
According to another exemplary aspect of the present invention, in one exemplary embodiment, each comparator of the analog to digital converter can have an individually adjustable threshold level. Further, each comparator can also have an externally controllable threshold level. With such individually adjustable thresholds, the comparators can process a multilevel signal comprising non-uniformly spaced decoding thresholds.
With this simple design, the analog to digital converter can reduce dissipated power and can increase achievable operational speeds for communications. Some unique properties of the Q-Gray coded multilevel signal that dictate the design of the aforementioned analog to digital converter are the following: The maximum number of bit-toggles incurred in a bit channel while sequentially traversing the code is minimized. An example of a situation in which the above minimum-maximum criterion is satisfied is when the Q-Gray code has a transition density that is maximally evenly distributed among bits in the binary representation of the multilevel signal. This maximally even transition distribution results in the property that the bit error rate is substantially evenly distributed across each bit channel.
The present invention provides for a method of high speed communications using an inventive Q-Gray code. In turn, the Q-Gray code simplifies the hardware needed to convert multilevel Q-Gray coded signals to binary signals.